How Earth seems quite unique

  1. What single observable (such as complexity) might characterize Earth as unlike any other planet in the universe?
     
  2. jcsd
    Earth sciences news on Phys.org
  3. The large moon, stabilizing it's spin axis, preventing Earth from getting into the chaotic zone
     
  4. Integral

    Integral 7,346
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    Since we have observed such a tiny percentage of planets in the universe we can only speculate about how unique we are.
     
  5. There's roughly 10^24 planets in the universe.

    No reason to suspect that ours is unique.
     
  6. turbo

    turbo 7,366
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    And some reason to suspect that all of them are unique in some way. We have a very small sample to observe here in our neighborhood, and there are some vast differences between them in many metrics. There is no reason to assume that other planetary systems would not have significant differences, as well. We have discovered extra-solar planets by lots of means, not the least of which is tracking light-curves to discover large planets partially occulting their stars when their orbits are aligned with our line-of-sight. Are we going to find a "Mercury" orbiting a large star that way? Probably not for a very long time.
     
  7. Are you sure this number isn't a substantial overestimate? First start with the visible universe. Take 10^11 stars per galaxy and say 5x10^11 galaxies (which would be much more than the ones we can see). So that's 5x10^22 stars. How many of those stars have planets? Most stars we know of are multiple star systems which are unlikely to have planets. It seems you might be over-estimating by a factor of at least 10^3. We really don't have enough information to estimate the number of planets in our galaxy, let alone the universe. The inner galaxy is a very different neighborhood compared to our location out in one of the spiral arms. I grant you, there are very likely many, many planets in the universe, but I wouldn't try to put a number on it. Estimates for Drake's equation are deliberately kept very conservative.
     
  8. Might such a large moon - having its genesis in the coalescing debris of a planetary impact - be more likely suited to avoid chaos by stabilizing the spin axis of the resultant planet?
     
  9. You've missed the point...

    Consider it 10^20+
     
  10. mgb_phys

    mgb_phys 8,952
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    Earth has both cyrus and evo - that must be pretty unique!
     
  11. Good one. Would every spinning (teristrial?) planet without substantial moon eventually enter the chaotic zone or not? This is caused by -as Laskar proposes- resonance between the obliquity cycle and the precession cycle. The moon speeds up the frequency of the precession cycle, preventing resonance, but what does it do to the obliquity cycle?

    A good thesis question?

    Second question of course is, how unique is that moon? Regardless of which hypothesis is true for the forming of Earth's moon, how feasible are these mechanims for forming similar planet - moon pairs elsewhere?
     
  12. Integral

    Integral 7,346
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    I am not sure what the deal is with the moon and stability? We have moonless Venus and Mercury, clearly a moon is not a necessary thing. Still we simply do not have the knowledge to talk about what is common or uncommon in the universe in general.
     
  13. D H

    Staff: Mentor

    Read section 2.2 of the paper that Andre cited. It is Earth's obliquity rather than its orbit that the Moon acts to stabilize. Lasker is claiming that Venus' rather anomalous rotation is due solely to gravitational effects by the other planets. The Moon acts to stabilize the Earth's rotation.
     
    Last edited: Jun 28, 2009
  14. negitron

    negitron 842
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    Without a large, stabilizing moon, Mercury, Venus and Mars all experience much larger variations in axial obliquity--Venus has turned itself nearly completely upside down! It's unclear as to whether a relatively stable axis is necessary for life but what is clear is that things here would be very different without our Moon. For one thing, seasonal variations would be far more extreme.
     
  15. But are Venus and Mercury anywhere near Earthlike? The only thing that Venus shares with Earth is the order of magnitude of its size

    And about the (r)evolution of it's rotation:

    Correia A.C.M., Laskar J., de Surgy O.N. (2003). "Long-term evolution of the spin of Venus: I. theory" (PDF). Icarus 163: 1–23
    ^ Correia A.C.M., Laskar J. (2003). "Long-term evolution of the spin of Venus: II. numerical simulations" (PDF). Icarus 163: 24–45

    which elaborates extensively that the lack of rotation of Venus is due to the chaotic zone, resonance between the obliquity cycle and precession cycle, causing excessive tilting of the spin axis, ultimately bringing the rotation of the planet to a halt. It's supposed to be the moon preventing the same fate for Earth.
     
  16. negitron

    negitron 842
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    No, but that wasn't at all my point.
     
  17. But it is the subject of the thread.
     
  18. negitron

    negitron 842
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    No, it's not.
     
  19. D H

    Staff: Mentor

    That was pithy. :rolleyes:

    The subject of this thread is the uniqueness of the Earth. Loren asked
    That was not the best way to phrase the question (in fact, I don't quite know what question Loren is asking here). Suppose the Earth does have a near twin somewhere in the universe, right down bearing humaniform intelligent life capable of sending spacecraft outside the planet's gravitational sphere of influence. Something will certainly distinguish our Earth from that near twin, even if it is milligrams in difference in planet mass or centimeters in difference in orbital distance from the parent star. Suppose the Earth has lots of such near twins. One may differ in mass, another will orbit at a slightly different radius, another will orbit a star with slightly different characteristics. There might be a lot of planets out there, but it is a finite number. Our Earth will always differ from its near-twins in some regard.

    I don't think that is what Loren was asking for. Asking for one single characteristic was part of the problem. Those who espouse the rare Earth hypothesis claim that the Earth's (relative) uniqueness results not from one characteristic but a whole slew of characteristics.


    That said, my answer to Loren's original question is that the Earth supports intelligent lifeforms capable of sending spacecraft outside the planet's gravitational sphere of influence.

    Is this characteristic unique throughout the universe? I doubt it. Whether it is rare is an entirely different question. If the closest planet to Earth that bears this characteristic is hundreds of thousands of galaxies away, does it really matter that we aren't unique? We are utterly, utterly alone. Does it really matter if the closest intelligent life-bearing planet is a lot closer, say ten galaxies away? Even in that case, we are still utterly alone, even though the Earth is far from "unique".
     
  20. The subject here is really whether there are any other planets having characteristics friendly to intelligent life as we know it. Any planet that we are likely to ever discover can be expected to be unique in some way. Key points:

    .If the planet is beyond any future limit of human exploration or discovery (whatever that might be), the question is irrelevant. However, "they" might be able to find us, in which case they might be kind enough to tell us about their home planet. Then again, they might not be so kind.

    .There might civilizations that have long since disappeared (if and when we find evidence of them) or that have flourished (or are flourishing) in the unseen void where the light has not yet reached us.

    .We might well find planets with life, but not intelligent life; which I take to mean having some kind of technology that we would recognize. These (non-intelligent) possibilities exist in our own solar system: on the Jupiter's moon Europa (covered with water ice, but with a vast liquid sea below), or the Jovian atmosphere at a level where liquid water exists at earth-like temperatures and pressures.

    .If we do discover a "twin earth" we might be way too early (or too late) to shake hands (or whatever) with an intelligent species. After all, intelligent life (as I defined it) is rare on earth. In the case of a technologically advanced species (which I define as being able to use electromagnetic devices for communication), this has existed on earth a very short time; since about 1844 compared to the 4.6 billion years that Earth has existed.

    Note: I've plagiarized much of this post from my own book. (see my member page if interested).
     
    Last edited: Jun 28, 2009
  21. D H

    Staff: Mentor

    I agree that that is the hidden agenda behind his original post. That is not what Loren asked in the original post. What Loren asked was "What single observable (such as complexity) might characterize Earth as unlike any other planet in the universe?"

    This is a silly question that verges on being a straw man. Only the most vehement of rare Earth proponents say the Earth is "unlike any other planet in the universe." Does it really matter whether there are hundreds of thousands versus just one Earth-like planets spread around the universe? The end result is the same: We are alone. The only way we are not alone is if there are hundreds of billions of Earth-like planets spread around the universe.
     
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